Hydraulic elevators typically comprise an elevator car, a sheave, a hydraulic cylinder, and a rope positioned inside a hoistway. One end of the rope is fixed to the car. From the car, the rope travels upward in the hoistway to the sheave. The sheave is attached to the rod end of the hydraulic cylinder. The rope wraps around the sheave and returns back down the hoistway, attaching at the other end to the bottom of the hoistway. The hydraulic cylinder propels the car through the hoistway indirectly by moving the sheave up and down the hoistway. Since the rope is a finite length, changing the position of the sheave changes the position of the car in the hoistway.
For safety reasons, elevators are generally required to have an overspeed governor and safeties. A typical overspeed governor includes a governor rope extending the length of the hoistway, attached to a sheave at the top of the hoistway and a governor sheave at the bottom of the hoistway. The governor rope is fixed to the elevator car by a linkage extending from the rope to a pair of safeties attached to the car. Because of the fixed relationship, the car and the rope are normally stationary relative to each other.
With this type of governor, if the downward velocity of the elevator exceeds a predetermined limit, i.e., an overspeed condition, a centrifugal flyweight assembly driven by the governor sheave swings outwardly and operates a governor brake. The governor brake applies a frictional drag force to the governor rope, thereby actuating a pair of safeties attached to the elevator car. The safeties act on a pair of guiderails and the car is consequently brought to a stop safely. Mastroberte U.S. Pat. No. 3 327,811 discloses such an arrangement.
This style overspeed governor, although effective, protects against overspeed conditions in one direction only. Specifically, a device like that disclosed by Mastroberte only works if the rope is traveling in a downward direction relative to the governor brake which acts on the rope. To use this type of governor, therefore, it is necessary to either position the governor sheave in a room at the top of the hoistway or to adopt a more elaborate roping scheme which would allow a Mastroberte type governor to be implemented in the pit of the hoistway. Neither of these solutions is very attractive, however. One of the great advantages of a hydraulic elevator is that no machine room is required at the top of the hoistway. Positioning a governor at the top of the hoistway would eliminate this advantage. A more elaborate roping scheme, on the other hand, would require additional sheaves and a longer rope. These additional items require greater initial cost, greater maintenance, and more clutter in the hoistway.